The present invention relates to a nitride semiconductor device such as a nitride semiconductor light emitting device applicable to, for example, a short wavelength light emitting diode or a blue-violet semiconductor laser and a nitride semiconductor photo detector device capable of receiving visible light or ultraviolet light.
GaN based group III-V nitride semiconductor (which will be hereafter referred to as merely “nitride semiconductor”) have a large bandgap, i.e., 3.4 eV at room temperature in GaN, and a pn junction or double hetero junction structure can be easily formed. For this reason, nitride semiconductor is applicable to light emitting devices such as a visible region light emitting diode and a short wavelength semiconductor laser. GaN based light emitting devices have been in practical use, and research and development for further improving device properties of GaN based light emitting devices have been intensively conducted. As for light emitting diode, blue, green and white light emitting diodes have been already introduced to the market. Also, as for semiconductor lasers, blue-violet semiconductor lasers have been developed as commercial products for use in next generation optical disk systems. To extend application fields of GaN based light emitting devices, longer luminescent wavelength of light emitting devices is one of technical challenges. For example, the luminescent wavelength larger than the wavelength in the green region is strongly desired.
In the history of development of GaN based light emitting devices, the improvement of device performance is largely dependent on crystal growth technology mainly by metal organic chemical vapor deposition (MOCVD). Specifically, progress of the material technologies such as hetero epitaxial growth using a low temperature buffer layer on a sapphire substrate, InGaN multiple quantum well active layers and low resistance p-type GaN growth with subsequent an activation annealing has contributed to the improvement of device performance. To enable longer wavelength of emitted light, it will be essential to improve the quality of the quantum well structure or propose a new active layer structure.
Hereafter, a blue light emitting diode, which is a known nitride semiconductor light emitting device using a sapphire substrate, will be described with reference to FIGS. 10 and 11 (see, for example, Japanese Laid-Open Publication No. 6-314822).
FIG. 10 is a cross-sectional structure of a light emitting diode using nitride semiconductor in the known nitride semiconductor device. FIG. 11 is an illustration of a band diagram in part of the light emitting diode located around a quantum well active layer in the known example. In FIG. 10, 301 denotes a C plane sapphire substrate, 302 denotes an n-type GaN layer, 303 denotes an n-type AlGaN cladding layer, 304 denotes an InGaN/GaN multiple quantum well active layer, 305 denotes a p-type AlGaN cladding layer, 306 denotes a Ti/Al electrode, 307 denotes a Ni/Au transparent electrode and 308 is an Au electrode.
Hereafter, a method for fabricating the light emitting diode of FIG. 10 will be described. First, an n-type GaN layer 302, an n-type AlGaN cladding layer 303, an InGaN/GaN multiple quantum well active layer 304 and a p-type AlGaN cladding layer 305 are formed in this order on a sapphire substrate 301, for example, by MOCVD. Subsequently, for formation of an electrode on the n-type layer side of this wafer, etching, specifically, dry etching using, for example, Cl2 gas is performed to the wafer to remove parts of the p-type AlGaN cladding layer 305, the InGaN/GaN multiple quantum well activation layer 304 and the n-type AlGaN cladding layer 303. Then, a Ti/Al electrode 306 is formed in the n-type layer side so as to be in contact with the n-type AlGaN cladding layer 303 and a Ni/Au transparent electrode 307 is formed on the p-type layer side. In this case, the thickness of the Ni/Au transparent electrode 307 has to be 10 nm or less so as to serve as a transparent electrode. Subsequently, to form a p-type layer side bonding pad, an Au electrode 308 is selectively formed on the Ni/Au transparent electrode 307. With use of a transparent electrode, for example, a large portion of blue emission light of 470 nm emitted from the InGaN/GaN multiple quantum well active layer 304 passes through the Ni/Au transparent electrode 307 and is drawn to the outside. The band diagram of active layer part of the light emitting diode structure of FIG. 10 is shown in FIG. 11. In FIG. 11, part of the wafer which is closer to an outer surface thereof is shown in the left hand side and part of the wafer which is closer to the substrate is shown in the right hand side. When the active layer of FIG. 11 is formed on a C plane, i.e., (0001) plane, spontaneous polarization occurs in GaN while piezoelectric polarization which is caused by crystal strain due to lattice mismatch and spontaneous polarization occur in InGaN (see, for example, S. F. Chichibu et al., Applied. Physics. Letters 73 (1998) pp. 2006-2008). Those polarizations cause the generation of an internal electric field in the active layer. As a result, the active layer has a structure in which electrons are accumulated in part of the InGaN layer which is closer to an outer surface of the wafer and holes are accumulated in part of the InGaN layer which is closer to the substrate.
However, in the known light emitting diode of FIG. 10, for example, when emission of long wavelength light such as green light (of 550 nm) is intended, an In composition of the InGaN layer used in a well layer for forming an active layer has to be, for example, about 35% or more. Thus, there arise problems with respect to crystal growth, such as crystal dislocations caused by lattice mismatch between the InGaN well layer and the GaN layer and In segregation in the InGaN well layer. Accordingly, crystal quality of the active layer is worsened and improvement of luminous efficiency is limited. Another problem is that due to the internal electric field, electrons and holes are spatially separated from each other in the InGaN well layer, so that luminous efficiency is reduced.